When exposed to radiation such as X-rays, an energy-storing phosphor (stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet having the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is then subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support (substrate) and a phosphor layer provided thereon. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer so as to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed, for example, by a gas phase-accumulation method. In the gas phase-accumulation method, a phosphor source is vaporized or sputtered so that the phosphor would be deposited and accumulated on a substrate to form a layer of the phosphor in the form of columnar crystals. The phosphor layer thus formed by the gas phase-accumulation method contains no binder and consists of only the phosphor, and there are gaps among the columnar crystals of the phosphor. Because of presence of the gaps, the stimulating light can stimulate the phosphor efficiently and the emitted light can be collected efficiently. Accordingly, the storage panel having the accumulated phosphor layer has high sensitivity. Moreover, since the gaps prevent the stimulating light from diffusing parallel to the layer, the storage panel can give a reproduced image of high sharpness.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. It is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
U.S. Patent Publication No. 2001/0010831 A1, JP-A-2004-85430 and JP-A-2004-257798 disclose a process for preparation of a radiation image storage panel. In the disclosed process, stimulable phosphors such as CsBr:Eu are vaporized, deposited and accumulated on a substrate under a medium vacuum (approx. 0.1 to 10 Pa) to form a phosphor layer.
The vapor-deposition procedure is conducted in an apparatus in which the substrate is placed at a position facing to an evaporation source. The evaporation source is heated to generate a stream of vaporized substance, which flows onto the surface of the substrate and then deposits and accumulates thereon. From the viewpoint of productivity, the deposition efficiency (a ratio of amount of accumulated substance/amount of evaporated substance) is preferably as high as possible. On the other hand, from the viewpoint of quality of the resultant storage panel, the deposited phosphor layer preferably has an even thickness. Moreover, in consideration of emission properties, it is also desired that the deposited phosphor be uniform and be in the form of good columnar crystals.
JP-A-5-70932 discloses a vapor-deposition apparatus comprising a covering plate equipped with a heating means. In the apparatus, an evaporation source is heated so that vaporized substance is deposited on a substrate facing to the evaporation source. The covering plate is provided so as to surround a space from the evaporation source to the position where the substrate is placed. In the vapor-deposition procedure, the apparatus is evacuated to a high vacuum of 10−4 to 10−5 torr (1.33×10−2 to 1.33×10−3 Pa). It is described that the covering plate is heated to re-vaporize a deposited phosphor source once deposited on the covering plate. Therefore the covering plate is equipped with a heating means, and thereby the deposition efficiency can be improved.
It has been now noted by the present applicants that a stream of gaseous product under high vacuum (at a vacuum degree lower than 0.1 Pa) generally diffuses but broadens linearly as the stream approaches the substrate, as shown in FIG. 3 (5: substrate, 7, 8: resistance heaters, 11, 12: diffusion preventing walls, 22: stream of gaseous product). In contrast, as shown in FIG. 2 (21: stream of gaseous product), the stream of gaseous product under medium vacuum (0.1 to 10 Pa) diffuses and spreads all over the space because a small amount of gas such as Ar gas still remains under medium vacuum. The gaseous product partly comes into collision with the remaining gas such as Ar gas before the stream reaches the substrate, and consequently scatters in the whole space between the evaporation source and the substrate. The deposition efficiency under medium vacuum is, therefore, even more serious than that under high vacuum.
In addition, it has been noted that the deposition efficiency is further serious particularly in the case where a phosphor layer of radiation image storage panel is formed by the vapor-deposition procedure. The phosphor generally comprises at least two components: matrix component and activator component. If the temperature at which one has a predetermined vapor pressure is very different from the temperature at which the other has the same vapor pressure (for example, the CsBr matrix component of CsBr:Eu phosphor has the vapor pressure of 1.33 Pa at 556 K, while the activator compound EuBr2 has that vapor pressure at 1013 K), the compound of the lower temperature (i.e., EuBr2 in the above example) shows an extremely poor deposition efficiency in the vapor-deposition procedure. As a result, the components of the deposited phosphor layer are often not uniform or very different from the designed ones, and accordingly a radiation image storage panel comprising thus-formed phosphor layer has low sensitivity.
It has been further noted that when the covering plate is heated to vaporize again the substance deposited on the plate, splashing is often caused although the deposition efficiency can be improved. The phosphor layer formed on the substrate in that way comprises a phosphor in the form of coarse columnar crystals or has a rough surface, and accordingly the reproduced radiation image is liable to have points defects.